Balancing equipment for four-wire circuits



A. B. CLARK AND 0. s. DEMAREST. BALANCING EQUIPMENT FOR FOUR-WIRE CIRCUITS.

' APPLICATION FILED JAN. 29. I919- Patented May 24, 192 1.

INVENTORLS' AiTORNEY lines.

UNITED STATES PATENT OFFICE.

ALVA B. CLARK AND CHARLES TO AMERICAN TELEPHONE AND YORK.

BALANCING EQUIPMENT FOR FOUR-WIRE CIRCUITS.

Specification of Letters Patent.

Patented May 2st, 1921.

' Application filed January 29, 1919. Serial No. 273 804..

means of a balanced repeater circuit such as a tour-wire circuit, two transmission lines for transmitting in opposite directions and each including oneay amplifiers are connected at their ends to terminal two-wire The terminal two-wire lines are balanced by means of artificial lines and it a sufficiently high degree of balance is pro- \ided it is possible to secure a gain in transmission between the terminal stations of the tour-wire circuit without causing singing to take place over said circuit. Any considerable change in the condition of the terminal two-wire circuit may result in'a degree of unbalance sutlicien't to cause singing to take place. The problem of associating monitor-. ing, talking and ringing equipment with the two-wire line consequently involves difliculties which it is the purpose of this invention to overcome.

In general, the object of the invention is, therefore. to provide means whereby when the condition of the terminal two-wire portion of a balanced repeater circuit is changed, the circuit may still be maintained sufficiently balanced to prevent singing.

A particular object of thcinvention is to provide an arrangement whereby a listening apparatus of an operators monitoring circuit may be associated with the terminal two-wire line ota balanced repeater system without causing appreciable unbalance.

A still turther particular object of the invention is to provide means whereby an operators talking equipment may be associated with the terminal two-wire line at one of the terminal stations of the tour-wire circuit without causing singing to take place.

Still another particular ()bjQCt ot the invention has reference to the provision of in the county of Kings and State.

means whereby a monitoring operator may apply ringing current to the terminal two wire circuit of a balanced repeater svstem without causing appreciable unbalance.

An additional particular object of the inventlon is to provide means whereby, when the two-wire line is disconnected from the two-wire terminal of the balanced repeater circuit, the balanced condition of the circuit is still maintained.

T hcse objects, as well as others more fully hereinafter appearing, are secured by means of the ar angements set forth in the tollowmg description which may be best understood by reference to the accompanying drawing, the figure of which is a diagraminatic illustration of a preferred embodiment ot' the invention;

Referring to thedrawing, the apparatus at one terminal station of a tourwire circuit isshown, the tour-wire circuit comprising a line L for transmitting in one direction to a distant station and a line L for transmitting in the opposite direction from the distant station. Each of these lines include one- *ay amplifiers (not shown) and are interconnected at their ends through a three winding transformer arrangement 10 to a terminal two-wire line such as L. The equipment at both terminal stations of the tour-wire circuit will be substantially identical and will conform to that shown in the drawing. An operators monitoring circuit M including a receiver 1?, transmitter T and controlling keys K K and K terminates in a twin plug P adapted to cooperate with a twin ack J whereby the monitoring circuit may be bridged across the two-wire line L. The two-wire line L is normally balanced by an artificial line or network N and in order that the operator may listen in on the circuit without disturbing the balance, the normal connection of the monitoring circuit includes a high impedance transformer (l which. when bridged across the line L, reduces the transmission over said line by such a small amount that no appreciable unbalance of the circuit results. In order that sutficient energy may affect the receiver R of the monitoring equipment, an amplifier A is provided which amplifies the currents to such a value as to produce a proper volume of sound in the receiver.

lVhcn the talking apparatus or the ringing apparatus of the monitoring equipment tion of the two-wire line is quite materially changed due to the relatively low impedance of said talking and ringing apparatus To prevent unbalance due to this condition, networks N and N are provided, the former being interpolated inthe two-wire portion or" the circuit between thepoint of connection of the monitoring equipment and the four-wire circuit and the latter net work being substituted for the usual artifl cial line N. The network N consists of series resistances and a shunt resistance so proportioned that the impedance of the portion of the circuit beyond the shunt resistance may vary considerably without the impedance of the whole combination differing materially from that of the resistance l For example, if the series resistance of the network N be made 1600 ohms and the shunt resistance be made 100 ohms and the resistance of the element N be made 1800 ohms, no appreciable degree of unbalance will result whatever may be the condition of the circuit beyond the network N If, for instance, the line L be entirely disconnected the effective impedance of the net work will be 2000' ohms. If, on the other hand, the shunt element of the network N be short circuited, the resultant impedance will still be 1600(ohms so that the maximum possible variation is 200 ohms above or be low the impedance of the element N.

In order to provide for situations which necessitate the disconnection of the line L from the four-wire circuit for testing or other purposes, the jack J is so arranged that by inserting an idle single plug in the right hand lialf thereof, the networks N and N will be associated with the four-wire circuit in order to maintain the balance thereof.

Further details of the apparatus may be sufiiciently understood from a description of the operation which is as follows:

Assuming that the operator desires to listen in on the circuit by means of the monitoring equipment M, the plug P is inserted in the jack J and the monitoring key K is operated; A circuit is completed over the sleeve contacts of the plug and jack from ground through the winding of marginal relay 11, over said contacts, over sleeve conductor 12 and over the normal contact of key K, through high resistance 13 to battery. The resistance 13 is sutliciently high to prevent the actuation of the marginal relay 11 so that no circuit changes take place as a result of the closing of this circuit.

' The actuation of key K, completes a circuit over the lower make. contact of said key for relay 14 which is energized and completes a filament circuit of amplifier A thereby rendering said amplifier operative.

The operators receiverR is now associated with the line L over a circuit including conductors 15, make contacts of key K conductors 16, autotransformer .17, amplifier A,

high impedance transformer Q, conductors 18, back Contacts of relay 19, tip contacts of plug 0 and jack J, and conductors 20. The primary winding of transformerO is bridged directly across the line L but the transformer is of such high impedance that Should the operator desire to talk over the two-wire line-L or over the four-wire circuit, keyK will be restored and the key K, will be actuated. By restoring key K, the receiver R is bridged about one half of one of the windings of the induction coil 21 of the operator s set, the transmitter T of said operators set being associated through said transformer with conductors 22. The actuation of key K closes at its lower contact a circuit from battery through low resistance 23, lower make contact to key K conductor 12, sleeve contacts of plug P and jack J, through the winding of marginal relay 11 to ground. Resistance 23 is sufliciently low to permit of the energization of marginal relay 11 which completes a circuit from ground over its front contact and through the windings of relays 24 and 25 to battery. Relay 11 also completes a circuit for the relay 26. The relays 24 and 25 at their front contacts open the r irect connection between the twowire termi nals 27 of the four-wire circuit andthe line L and interpolate the network N Relay 26 disconnects the artificial line N. and substitutes the balancing element N in its stead.

The actuation of key K also closes the circuit from ground over the upper make contact of said key, over conductor 29 and through the winding of slow acting relay 19 to battery. As soon as relay 19 operates a circuit will be completed in parallel with the shunt resistance element of network N as follows:

Over the front contacts of relay 2%, back contacts of relay 2S, conductors 20, tip contacts of jack J and plug P, front contacts of relay 19, normal contacts of key K conductors 30, make contacts of key K and over conductors 22 to the operators set including transmitter T and receiver R. r

The parallel circuit above traced is of comparatively low impedance but as has been already explained the series and shunt elements' of the network N are so proportioned It will be noted with reference to the operationsjust described, that the relay 19 is slow acting so that it permits relays 24c, 25

and 26 to operate and connect networks N and .N in: circuit before-the low impedance 'operators set is bridged across the two-wire line. This prevents the possibility of a singing condition being set up due to the fact that the operators set is bridged across the two-wire line before the relays 24, 25 and'26 have had time to operate and connect the networks N,, and N in circuit.

Should the operator desire to transmit a ringing signal either'over the four-wire circuit to the distant terminal station thereof or 'over the two-wire 'line L to the station in which said line terminates, the ringing key K will be operated. A circuit is completed over the upper make contact of said key from battery through the low resistance 23, over conductor 31, make contact of key K to point 32, thereby completing a low resistance path to the sleeve conductor 12 in parallel with the high resistance path through resistance 13. The current fiow over sleeve conductor 12 and through the'winding of relay 11 is thereby increased so that marginal relay 11 is actuated and completes circuits for relays 2-1, 25 and 26 as previously described. nect networks N and N into the circuit in a manner already set forth. Key K, at its Y lower make contact also completes a circuit for the slow acting relay 19 which, after an interval, completes a circuit ior ringing current over the inner make contacts of key K front contacts of relay 19, tip contacts of plug P, ack J and over conductors 20 to points 33 and 3t at which points thecurrent divides, part flowing over the line L to the distant station and part flowing over the back contacts of relay 28, front contacts of relay 24:, through network N front contacts of relay 25, overconductors 27 through the bridge 35 and through the winding of ringing responsive relay 36. The current flowing over the line L may actuate ringing responsive means at the distant station to call the attention of the operator at said station. The current flowing through the relay 36 actuatcs said relay to open the normally closed circuit of relay 37 which, upon releasing its armature completes a circuit for relays 38 and 39. Relay 39 disconnects the autotransformer a0 and ringing responsive relay 411' Relays 24:, 25 and 26 operate to confrom bridge 35 to prevent the actuation of rmging responsive relay 41. Relay 38 at its lower front contacts completes a circuit for the generating apparatus 42 and at'its upper front contact completes a circuit for the relay 43' whereby ringing current from the generating apparatus 42 is transmitted through the transformer 14: and over the front contacts of relay 43 to the line L The ringing current transmitted over the line L upon arriving at the distant station operates ringing responsive apparatus at said station in a manner which will be presently made clear in connection with the description of the operations resulting from the transmission of ringing current from said distant station to actuate similar ringing responsive apparatus at the station illustrated in the drawing.

The connection of the relatively low-im-v pedance ringing path over the make contact key K to the line L does not cause the four-wire circuit to sing for the reason that said low impedance path is connected in parallel with the shunt element of network N so that a fair degree of balance is maintained in a manner similar to that described in connection with the operators talkingapparatus.

Should ringing current be transmitted from the distant station of the four-wire circuit over the line L said ringing current would flow through the autotranst'ormer 40 and through the windings 0t ringing responsive relay 41 to actuate said relay. Relay 41 would thereupon open the circuit of relay 15 which, in turn, completes the circuit of relays 46 and 39. Relay 39 disconnects the autotranstormer and relay 4C1 from the bridge thereby preventing the' current flowing from the line L from being transmitted to the line L Relay 4:6 at its upper front contact completes circuits for relays 24-, 25 and 26 thereby interpolating the network N in the line L and substituting the balancing element N for the artificial line N. Relay 46 at its lower front contact completes a circuit from ground over the upper front contact of relay 25 and through the winding of relay 28. Relay 2S disconnects the twowire line L from the four-wire circuit and connects ringing current to the two-wire line over the outer front contacts of relay 2 1 and over the inner front contacts of relay 28 so that ringing current is transmitted to the station at which the line L terminates. The unbalance of the four-wire circuit which would ordinarily be produced by the disconnection of the two-wire line L from the tour-wire circuit at the contacts of relay 28 is prevented by reason of the fact that networks X and N have been previously connected in circuit. Singing over the tour-wire circuit is, therefore, prevented during the ringing operation.

It is sometimes necessary to disconnect the two-wire line from the tour-wire circuit for purposes of testing or makmg repairs. bhould the two-wire line L be disconnected from the four-wire circuit at switching points 47 and 4:8 for instance, singing of the four-wire circuit may be prevented by pre viously inserting an idle plug in the right hand half of the. twinjack J. This operation would complete a circuit from battery through resistance a9, make contact 50, normal contact 51 and through the winding of marginal relay 11 to ground. Marginal relay 11 would be energized by the circuit thus completed and would operate relays 25 and 26 to connect networks N and N in circuit to balance the four-wire system.

By means of the arrangements above described, a simple and efiicientmeans is prorided for maintaining a reasonable degree of balance of a four wir'e circuit regardless of changes in the condition of the equipment connected to the two-wire terminals of the circuit and while the invention has been illustrated in what is thought to be its most desirable form, it will be obvious that the general principles thereof may be embodied in. many organizations widely different from that'disclosed without departing from the spirit of the invention as defined in the following claims:

What is claimed is:

1. In a signaling system, a repeater circuit, terminal two-wire lines connected to the repeater circuit, means for balancing said twowire lines, an operators listening set, means for connecting said listening set with one of said two-wire lines, and a high impedance transformer in said connection'to prevent appreciable unbalance of the repeater circuit.

2. In a signaling system, a repeater circuit, terminal two-wire lines connected to the repeater circuit, means for balancing said twowire lines, an operators listening set, means for connecting said listening set with one of said two-wire lines, a high impedance element in said connection to prevent appreciable unbalance of the repeater circuit, and an amplifier for amplifying currents received through said high impedance.

' 3. In a signaling system, a repeater circuit,

terminaltwo-wire lines connected to the repeater circuit, means for balancing said twowire lines, an operators talking set of low impedance, means for connecting said set with one of said twowire lines, and means responsive to such connection to associate auxiliary balancing equipment with the circuit to prevent appreciable unbalance of the repeater circuit.

I. In a signaling system, a repeater circuit, terminal two-wire lines connected to the repeater circuit, means for balancing said twowire lines, an operators talking set of low impedance, means for connecting said set with one of said two-wire lines, and means responsive to such connection to include an means responsive to such connection to include an auxiliary network in the two-wire line between the talking set and the repeater circuit and to substitute an auxiliary balancing means or said first balancing means in order to prevent appreciable unbalance of the repeater circuit.

G. In a signaling system, a repeater circuit, terminal two-wire lines connected to the repeater circuit, artificial lines for bill:

ancing said two-wire lines, an operators equipment of low impedance, a key for con trolling the connection of said equipment w1th one of said two-wire lines, means responsive to the action of said key for including an auxiliary network in the twowire line between the point of connection of the operators equipment and the repeater circuit to prevent appreciable unbalance of the repeater circuit, and to thereafter connect the operators equipment with the twowire line.

7. In a signaling system, a repeater circuit, terminal two-wire lines connected to the repeater circuit, means for balancing said two-wire lines, an operators ringing equipment, means for connecting said equipment with one of said two-wire lines, and means responsive to such connectionto associate auxiliary balancing equipment with the circuit to prevent appreciable unbalance of the repeater circuit.

8. In a signaling system, a repeater circuit, terminal two-wire lines connected to the repeater circuit, means for balancing said two-wire lines, an operators ringing equipment, means for connecting said equipment with one of said two-wire lines, and means responsive to such connection to include an auxiliary network in the two-wire line between the point of connection and the repeater circuit to prevent appreciable unbalance of the repeater circuit.

9. In a signaling system, a'repeater circuit, terminal two-wire lines connected to the repeater circuit, means for balancing said two-wire lines, means to disconnect one of said two-wire lines from'the repeater circuit, and means whereby an auxiliary balanci'ng means may be connected to the twowire terminal of the repeater circuit when the two-wire line is disconnected.

\ 10. A transmission circuit including lines interconnected by repeater apparatus, means to balance said line's, an operators listening set, means for connectingsaid listening set with one of said lines, and a high impedance transformer in said connection to prevent 5 appreciable unbalance of said-line.

11. A transmission circuit including lines interconnected by repeater apparatus, means to balance said lines, an operators listening set, means for connecting said listening set 10 with one of said lines, a high impedance element in said connection to prevent appreciable unbalance of said line, and an amplifier for amplifying currents received through said high impedance.

In testimony whereof we have signed our 15 names to this specification this twentyeighthday of January, 1919} i ALVA B. CLARK. CHARLES s. DEMAREST. 

